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Derivative Financial Instruments and Hedging Activities - Narrative (Details) - Fair value hedge - Designated as hedging instrument $ in Thousands	Jun. 30, 2021 USD ($) derivative	Dec. 31, 2020 USD ($)
Derivatives, Fair Value [Line Items]
Interest rate swaps accrued interest | $	$ 10	$ 11
Interest rate swap
Derivatives, Fair Value [Line Items]
Number of instruments held | derivative	4

X

- Definition

Derivative, Accrued Interest, Interest Rate Swaps

+ References

No definition available.

+ Details

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Namespace Prefix:	bmrc_
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Balance Type:	credit
Period Type:	instant

X

- Definition

The number of derivative instruments of a particular group held by the entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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- Details

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- Details

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